Since the metal organic chemical vapor deposition (hereinbelow abbreviated to MOCVD) method, which is a thin film growth method using metal organic material, is excellent in mass productivity and controllability as a method for growing compound semiconductor thin film, etc., has been widely utilized. By the reduced pressure MOCVD method, by which thin film is grown in a reactor with a reduced pressure, among others, since gaseous phase nucleus formation, which is a problem to be solved for the atmospheric pressure method, is small, the quality and the uniformity of the thin film are improved. Therefore recently research and practical use thereof have been extensively carried out.
In the case where e.g. III - V compound is grown by this reduced pressure MOCVD method, it is usual to use organic metal as III group material and hydride as V group material. More concretely speaking, in the case where an AIN film is grown, TMA (trimethylaluminium) is used as III group material and NH.sub.3 as V group material. Introduction of TMA into the reactor for the thin film growth is effected by introducing carrier gas such as H.sub.2 into TMA (which is liquid at the room temperature) while effecting bubbling. In this way TMA gas corresponding to the saturated vapor pressure at the bubbler temperature is introduced by the carrier gas into the reactor.
Representing the flow rate of the carrier gas in mol by Q.sub.c (mol/min), the flow rate of TMA gas by Q.sub.TMA, the pressure within the bubbler by P.sub.b (Torr), and the saturated vapor pressure of TMA at the bubbler temperature T.sub.b by P.sub.TMA (Torr), the flow rate of TMA gas in mol Q.sub.TMA can be given by the following equation; ##EQU1##
FIG. 7 illustrates the construction of the metal organic gas supplier in an atmospheric pressure type MOCVD device. In the figure reference numeral 1 indicates a bubbler made of stainless steel, 2 organic metal (TMA), 3 a stop valve disposed at the inlet of the bubbler, 4 a stop valve disposed at the outlet of the bubbler, 5 a bypass valve, 6 a mass flow meter, 7 a flow path, 8 a reactor, 9 a substrate and 10 a thermostat.
The flow rate of the carrier gas can be controlled usually with a high precision by means of the mass flow meter 6, etc. Further, since the temperature regulation of the bubbler 1 and the organic metal (liquid) 2 can be controlled with a high precision by means of the thermostat 10, P.sub.TMA can be also regulated easily. Since the pressure in the reactor 9 is 1 atm (760 Torr), the pressure in the bubbler 1 is higher than that in the reactor by the pressure loss in the flow path 7 between the reactor 8 and the bubbler 1. However, at the flow rate of the carrier gas usually used (several hundreds.about.several thousands cc/min) the pressure loss in the flow path 7 of the vapor between the reactor 8 and the bubbler 1 is negligibly small. Consequently the pressure in the bubbler 1 is approximately equal to the pressure in the reactor 8, which remains always to be 1 atm. As it is clear from Eq. (1), Q.sub.TMA can be stabilized relatively easily by the atmospheric pressure CVD method. FIG. 8 indicates the pressure distribution within an atmospheric pressure MOCVD device.
On the contrary, FIG. 9 represents the construction of an apparatus according to the reduced pressure method. In the figure the reference numerals, which are used in common in FIG. 7, represent items identical or corresponding to those indicated in FIG. 7 and 11 indicates a pressure difference generator. FIG. 10 is a graph indicating the pressure distribution within an reduced pressure MOCVD device, which corresponds to that indicated in FIG. 8.
It differs from the atmospheric pressure method in that a pressure difference generator 11 is disposed between the reactor 8 and the bubbler 1. Usually a variable needle valve is used as the pressure difference generator 11. Representing the pressure in the reactor by P.sub.6, the bubbler pressure P.sub.3 can be given as follows; EQU P.sub.3 =P.sub.6 +(P.sub.5 -P.sub.6)+(P.sub.4 -P.sub.5)+(P.sub.3 -P.sub.4)
At the usual flow rate, since the pressure losses (P.sub.3 -P.sub.4) and (P.sub.5 -P.sub.6) are very small with respect to (P.sub.4 -P.sub.5), the following equation is valid; EQU P.sub.3 .apprxeq..sub.6 +(P.sub.4 -P.sub.5) (2)
and the reactor pressure .sub.6 can be represented by EQU P.sub.6 =Q.sub.t /S.sub.r
in which Q.sub.t indicates the total flow rate of the gas flowing in the reactor and S.sub.r The effective evacuation speed at the outlet of the reactor. Q.sub.t can be stabilized by controlling the gas flow rate by means of a mass flow meter. Further S.sub.r can be stabilized by using a vacuum pump whose fluctuations in the evacuation speed are small. Consequently the value of P.sub.6 can be considered usually to be stable. Therefore it can be understood from Eq. (2) that it is the pressure loss (P.sub.4 -P.sub.5) in the pressure difference generator portion that influences most strongly on P.sub.3 (=P.sub.6).
When the conductance in the pressure difference generator portion 11 is kept constant (the opening ratio of the needle valve is constant), (P.sub.4 -P.sub.5) varies, depending on the viscosity, the flow speed, the specific weight, etc. of the fluid. Consequently, in the case where the fluid flowing through the pressure difference generator portion changes, when the viscosity and the specific weight vary considerably from those before the change, (P.sub.4 -P.sub.5) varies at the same time. Variations in (P.sub.4 -P.sub.5) provoke variations in P.sub.3 (=P.sub.6) and thus the value of Q.sub.TMA given by Eq. (1) varies also. Due to this fact the predetermined growth speed can be obtained no more and the controllability is worsened.
The operation for growing thin film will be explained by taking FIG. 9 as an example. At first carrier gas is supplied through the bypass valve 5, the pipe 7 and the pressure difference generator portion 11 to the reactor 8 with a predetermined flow rate by means of the mass flow meter. At this time the pressure difference generator portion 11 is so regulated that the pressure in the bubbler 1 is 1 atm (.apprxeq.760 Torr). The pressure distribution when a stationary state is obtained in this way is indicated by a full line in FIG. 10. During this operation the valves 3 and 4 are closed.
Next the valve 5 is closed and the valves 3 and 4 are opened one after another. In this way carrier gas is introduced into the bubbler 1 and the bubbling of TMA begins. Vaporized TMA gas is led through the flow path 7 and the pressure difference generator 11 to the reactor 8. At this time, since the gas passing through the pressure difference generator portion 11 changes from H.sub.2 100% to a mixture gas of H.sub.2 and TMA, remarkable variations in the viscosity and the specific weight of the gas are produced. It is also conceivable that TMA is adsorbed by the inner surface of the pipe and the pressure loss in the pressure difference generator portion 11 varies also. The concentration of TMA in the bubbler is usually about 1%, but the density thereof is more than 70 times as great as H.sub.2. Since the pressure loss .DELTA.P in the pressure difference generator portion 11 can be represented by EQU .DELTA.P=(P.sub.4 -P.sub.5).varies. .rho.
in which .rho. indicates the density, even if the concentration of TMA in the mixture gas passing through the flow path is only about 1%, increase in the pressure difference .DELTA.P =(P.sub.4 -P.sub.5) cannot be neglected. As the result, since P.sub.3 (=P.sub.6) increases also, the flow rate of TMA in mol (effective supplied amount) decreases, as it is understood from Eq. (1).
Variations in the pressure can be produced by other factors and the pressure in the bubbler can be negative. However, in any case, in order to stabilize the supplied amount of TMA, it is necessary to stabilize P.sub.3 (=P.sub.6).
FIG. 11 shows variations in the flow rate of organic metal (TMA) produced by the prior art reduced pressure MOCVD method in mol with the passage of time. It can be seen that the flow rate of TMA in mol have not the expected value, because of the fact that the pressure in the bubbler differs before and after TMA begins to flow (FIG. 12).
The pressure loss (.DELTA.P) in the flow path is influenced remarkably by the flow speed (v), i.e. EQU .DELTA.P.varies.V.sup.2.
Consequently even slight variations in the flow rate (and thus the flow speed) of the carrier gas produce large variations in the pressure loss in the pressure difference generator portion 11.
For the multi-layer growth of mixt crystal semiconductors such as Ga.sub.1-x Al.sub.x As, since the growth rate differs for every layer, it is necessary to vary the ratio of the supplied materials Al and Ga with the growth of every layer. In this case, for the atmospheric pressure system, since the bubbling pressure of the organic metal is constant (=760 Torr) independently of the flow rate, the control of x is effected simply by varying the ratio of the flow rate of the carrier gas. On the contrary, for the reduced pressure system, since variations in the flow rate provoke variations in the pressure loss in the pressure difference generator portion and thus variations in the bubbling pressure, according to the prior art techniques it was difficult to control the flow rate in mol of the organic metal with a high precision.
As explained above, according to the prior art reduced pressure method, since it was not taken into account that the pressure loss in the pressure difference generator portion 11 depends on the kind of the gas flowing therethrough, unstable variations in the pressure within the bubbler 1 were produced and the controllability of the organic gas supply was bad. Since originally by the MOCVD method the rate is determined by the transportation of material, the method has a great advantage that the growth speed is determined only by the flow rate of the organic gas and thus it is a method, which is excellent in the mass productivity and the controllability. However, under a reduced pressure, heretofore, since the MOCVD method had a problematical point that the pressure in the bubbler 1 fluctuated, there was a drawback that its advantage was not utilized sufficiently.
In addition the measurement of Q.sub.TMA was effected by mass-analyzing gas sampled from the reactor.